1. Field of the Invention
The present invention relates generally to x-ray systems and devices. More particularly, embodiments of the invention concern an x-ray device shield structure that facilitates control of problems such as gas arcing and heat concentrations.
2. Related Technology
X-ray systems and devices are valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing.
While used in a number of different applications, the basic operation of x-ray devices is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. A typical x-ray device includes an x-ray tube having a vacuum enclosure collectively defined by a cathode cylinder and an anode housing. An electron generator, such as a cathode, is disposed within the cathode cylinder and includes a filament that is connected to an electrical power source such that the supply of electrical power to the filament causes the filament to generate electrons by the process of thermionic emission. The anode is disposed in the anode housing in a spaced apart arrangement with respect to the cathode. The anode includes a target surface oriented to receive electrons emitted by the cathode. Typically, the target surface is composed of a material having a high atomic number so that a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, namely, x-rays.
In operation, the electrons are rapidly accelerated from the cathode to the anode under the influence of a high potential between the cathode and the anode that is created in connection with a suitable voltage source. The accelerating electrons then strike the target surface, sometimes referred to as a “focal track,” at a high velocity. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray device for penetration into an object, such as a the body of a patient. The x-rays that pass through the object can then be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
A relatively large percentage of the electrons that strike target surface of the anode do not cause the generation of x-rays however and, instead, simply rebound from the target surface. Such electrons are sometimes referred to as “back-scatter” electrons. In some x-ray tubes, some of these rebounding electrons are blocked and collected by an electron collector that is positioned between the cathode and the anode so that rebounding electrons do not re-strike the target surface of the anode. In general, the electron collector thus prevents the rebounding electrons from re-impacting the target anode and producing “off-focus” x-rays, which can negatively affect the quality of the x-ray image.
Typically, such electron collectors define an aperture through which the emitted electrons pass from the cathode to the target surface of the anode. To this end, the aperture includes or defines an inlet positioned near the cathode, as well as an outlet positioned near the target surface of the anode. In at least one implementation, the aperture is configured so that the inlet has a diameter that is relatively larger than the diameter of the outlet.
While such electron collectors have proven useful in some applications, some problems nonetheless remain. For example, the relatively close proximity between the small diameter outlet of the aperture and the target surface of the anode sometimes results in undesirable heat concentrations at the outlet. Such heat concentrations can cause, among other things, thermal stress and strain that may ultimately contribute to structural failure of the collector. More particularly, non-uniform thermal expansion of structural elements, such as is produced by high temperature differentials, induces destructive mechanical stresses and strains that can ultimately cause a mechanical failure in the part. Further, because the inlet of typical electron collectors is relatively larger than the inlet, backscattered electrons can readily rebound through the inlet and strike the cathode, thereby damaging the cathode and/or interfering with the electrons emitted from the cathode.
Yet other concerns with some typical electron collectors relate to anomalous current flows, such as arcing, within the x-ray device. More particularly, outgassing of metal and glass x-ray device components is generally employed to remove gases adsorbed to the surfaces of those components. The removal of these gases enables a relatively higher vacuum to be achieved in the evacuated enclosure of the x-ray device. In general, outgassing involves heating the x-ray device components to a high temperature for a predetermined period of time. However, typical outgassing processes do not remove all of the adsorbed gas, and some gases, whether present on or under the surfaces of such components, often remain even after outgassing has been performed. As discussed below, these remaining gases, as well as gases that may be produced during normal x-ray device operations, tend to accentuate certain shortcomings associated with typical electron collectors.
In particular, it was noted earlier herein that typical electron collectors are arranged with the large diameter portion of the aperture located near the cathode. Thus, a relatively large portion of the electron collection surface, where adsorbed gases are commonly present, is located in relatively close proximity to the cathode, where the electrical field strength is at or near a maximum. When an exposure commences, adsorbed gases tend to desorb, or ionize, as a result of the heat generated. The presence of the ionized gas in the strong electrical field near the cathode results in gas arcing and/or other anomalous current flow in the high field region. Among other things, such current effects compromise the performance and service life of the x-ray device and can damage or destroy the components of the x-ray device.
In view of the foregoing, and other, problems in the art, what is needed is a shield structure that at least partially attenuates heat concentration problems associated with some known electron collectors. In addition, the shield structure should contribute to a reduction in anomalous current flows within the x-ray device.